Electrical component testing apparatus having a temperature-compensating circuit

ABSTRACT

In testing apparatus in which voltages are developed across an electrical component and a standard resistance and are compared to determine whether the resistance of the electrical component falls within desired limits, temperature-compensating circuitry is provided for modifying the voltage developed across the standard resistance in response to changes in temperature and as a direct function of the temperature coefficient of resistance of a specific electrically conducting material in the electrical component. The temperature compensating circuitry is adjustable over a preselected range in accordance with the percentage of the specific electrically conducting material in the electrical component, and is connected between the standard resistance and a comparator circuit so as to feed the modified voltage to the comparator circuit for comparison with the voltage developed across the electrical component.

United States Patent Primary Examiner-Edward E. Kubasiewicz AttomeysH.J. Winegar, R. P. Miller and A. C. Schwarz, Jr.

ABSTRACT: In testing apparatus in which voltages are developed across anelectrical component and a standard resistance and are compared todetermine whether the resistance of the electrical component fallswithin desired limits, temperature-compensating circuitry is providedfor modifying the voltage developed across the standard resistance inresponse to changes in temperature and as a direct function of thetemperature coefficient of resistance of a specific electricallyconducting material in the electrical component. The temperaturecompensating circuitry is adjustable over a preselected range inaccordance with the percentage of the specific electrically conductingmaterial in the electrical component, and is connected between thestandard resistance and a comparator circuit so as to feed the modifiedvoltage to the comparator circuit for comparison with the voltagedeveloped across the electrical component.

PATENTEUJAN 4312 l 3.333.093

\NVENTOQ D.\ .WESTLUND ELECTRICAL COMPONENT TESTING APPARATUS HAVING ATEMPERATURE-COMPENSATING CIRCUIT BACKGROUND OF THE INVENTION 1. Field ofthe Invention V This invention relates to apparatus for testingelectrical components, and more particularly to apparatus for testingthe resistance of an electrical component in which the percentage of aspecific electrically conducting material in the electrical componentmay vary and in which the environment in which the electrical componentsare being tested is subject to temperature change.

2. Description ofthe Prior Art In the manufacture of certain electricalcomponents, such as coils for electromechanical relays used in telephoneswitching systems, it is standard practice to test the direct cur rentresistance of each coil to ascertain whether it is equal to a desiredstandard value within permissible tolerance limits. Generally, the testis performed in the factory at the ambient or room temperature bycomparison-type apparatus in which a voltage is developed across thecoil and is compared with a voltage which is developed across a standardresistance.

Frequently, the same test apparatus is utilized for testing a number ofdifferent types of coils which are constructed ofdifferent percentagesof copper and which therefore have different temperature coefficients ofresistance. Accordingly, since the ambient temperature in the factorytends to vary considerably it is necessary to compensate for this factin order to obtain accurate test results.

Heretofore, this has been accomplished in a number of different ways.For example, it is standard practice to provide a different standardresistance for each type of coil and to construct the standardresistance of the same material as its associated coil, therebyminimizing errors due to changes in ambient temperature. This procedure,however, is undesirable where a large number of different types of coilsare to be tested because of the large number of standard resistancesrequired. Further, this procedure is inaccurate, particularly when alarge number of coils are tested in succession using the same standardresistance, because while each coil tested is initially at roomtemperature, the standard resistance becomes successively warmer witheach test, whereby its resistance progressively changes according to itstemperature coefficient of resistance.

It also is common practice to test the resistance of coils utilizing abridge circuit, one branch of which includes a coil under test andanother branch of which includes a standard resistance in series with atrimmer resistance. The standard resistance and the trimmer resistanceboth have a low or near zero temperature coefficient of resistance andthe trimmer resistance is calibrated in degrees Fahrenheit to compensatefor the change in the resistivity of copper with changes in ambienttemperature. In testing a series of a particular type of coil in thissystem the operator initially reads a precision thermometer to obtainthe ambient temperature and adjusts the temperature-compensatingresistance accordingly. Subsequently, as the testing of the coilsproceeds, the operator periodically refers to the thermometer andadjusts the temperature-compensating resistance in the bridge asnecessary.

This system is undesirable because the ambient temperature may changebetween the times that the operator checks the temperature, or theoperator may neglect to make the periodic temperature checks and thenecessary corrections on the ternperature-compensating resistance. Thesystem also is subject to error on the part of the operator in makingthe periodic temperature readings and resistance settings.

SUMMARY OF THE INVENTION An object of this invention is to provide newand improved apparatus for testing electrical components, in whichvariations in ambient temperature are compensated for automatically.

In accordance with the invention, voltages are developed across anelectrical component and a standard resistance and the voltage developedacross the standard resistance is fed to a temperature-compensatingmeans for modifying the voltage in response to changes in temperatureand as a function of the temperature coefficient of resistance of aspecific electrically conducting material in the electrical component.The temperature-compensating means is adjustable over a preselectedrange in accordance with the percentage of the specific elec' tricallyconducting material in the electrical component and is connected betweenthe standard resistance and a comparing means so as to feed the modifiedvoltage to the comparing means for comparison with the voltage developedacross the electrical component.

More specifically, the temperaturecompensating means includes twoamplifying circuits connected in parallel and both driven by the voltagedeveloped across the standard resistance. The first amplifying circuithas a constant gain and therefore an output voltage which is in directproportion to the voltage developed across the standard resistance. Thesecond amplifying circuit has a gain which varies in response to changesin ambient temperature and as a direct function of the temperaturecoefficient of resistance of the specific electrically conductingmaterial, and accordingly has an output voltage which is proportional tothe voltage developed across the standard resistance as a directfunction of the temperature coefficient of resistance of the specificelectrically conducting material. Adjustment of thetemperature-compensating means in accordance with the percentage of thespecific electrically conducting material in the electrical component isaccom plished by a voltage tap and a resistance connected across theoutputs of the amplifying circuits such that the voltages at successivepoints on the resistance vary as a function of the temperaturecoefficient of resistance of the specific electrically conductingmaterial, from an optimum voltage which is representative of the optimumresistance for an electrical component containing zero percent of thematerial to an optimum voltage which is representative of the optimum resistance for an electrical component containing percent of the material.Buffer amplifying means is provided for feeding the voltage at thevoltage tap to the comparing means and for isolating the voltage tapfrom the comparing means.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows an electrical circuitfor test apparatus in accordance with the invention.

DETAILED DESCRIPTION Referring to the drawing, the disclosed embodimentof the invention is designed for testing the direct current resistanceof relay coils 11, one of which is illustrated schematically in thedrawing as a resistance under test, to determine whether the resistanceof the coils is equal to that of a desired standard within certainlimits. The apparatus is specifically designed for testing relay coilsll of different types, wherein each type of coil is constructed of amaterial containing a different percentage of an electrically conductingmaterial, such as copper, and thus has a different temperaturecoefticient of resistance.

The apparatus includes a standard resistor 12, which is of a precisiontype having a zero temperature coefficient of resistance and which isadjustable to a resistance value equal to an optimum resistance for thecoils llll being tested. The standard resistor 12 and contacts 13 and 14of a suitable receptacle (not shown) for receiving one of the coils 11during a test operation are connected in series across a battery 16through respective terminals 17 and I8, and a junction terminal 19between the standard resistor and the adjacent contact 13 isgrounded.'Thus, when one of the coils 11 is inserted in the receptaclefor testing, as illustrated in the drawing, since the same current flowsthrough the coil and the standard resistor 12, the voltage at each ofthe terminals I7 and I8 is ofa magnitude which is proportional to thevalue of the standard resistor and the resistance of the coil,respectively.

in the illustrated embodiment of the invention, the voltage developedacross the standard resistor 12 and appearing at the terminal 17 is fedto a temperature-compensating circuit 21, which modifies the voltageautomatically in response to changes in ambient temperature and as afunction of the temperature coefficient of resistance of copper. Thismodified voltage then is fed to a comparator circuit 22 for comparisonwith the voltage developed across the coil 11 under test and appearingat the terminal 18, to determine whether the resistance of the coil iswithin acceptable limits.

The temperature-compensating circuit 21 includes two amplifying circuits23 and 24 connected in parallel and both driven by the voltage appearingat the terminal 17. The first amplifying circuit 23 includes an inputresistor 26 and a direct current amplifier 27 having a feedback loopgiving it a gain of exactly minus one. In this connection, the feedbackloop includes a feedback resistor 28 and an adjustable resistor 29 whichmay be used to compensate for normal tolerance deviations in theresistance values of the resistors 26 and 28 and to adjust the gain ofthe amplifier to exact unity. The resistors 26, 28 and 29 all have azero temperature coefficient of resistance. Thus, the output voltage ofthe amplifier 27 is equal in magnitude to the voltage developed acrossthe standard resistor 12 and appearing at the terminal 17, but ofopposite polarity.

The second amplifying circuit 24 includes a direct current amplifier 31having a normal gain of minus one, but having a feedback loop whichincludes only a resistor 32 of pure copper. The amplifying circuit 24also includes adjustable input resistors 33 and 34, the resistor 34being used for fine adjustment and having a resistance value ofapproximately 1 percent of that of the resistor 33. The input resistors33 and 34 both have a zero temperature coefficient of resistance, and inoperation are adjusted so that their combined effective resistancematches the resistance of the pure copper feedback resistor 32 at afixed reference temperature, such as 68 F. Accordingly, the gain of theamplifier 31 varies with ambient temperature as a function of thetemperature coefficient of resistance of copper, and the output voltageof the amplifier is proportional to the voltage developed across thestandard resistor 12 and appearing at the terminal 17 as a directfunction of the temperature coefficient of resistance of copper, but ofopposite polarity.

The temperature-compensating circuit 21 also includes a linearproportioning resistor 36 which is connected across the outputs of theamplifiers 27 and 31. Thus, the voltages on successive points of theresistor 36 vary as a function of the temperature coefficient ofresistance of copper, from an optimum voltage which should be developedacross the coil 11 under test if it contains zero percent copper, at aterminal 37, to an optimum voltage which should be developed across thecoil if it contains 100 percent copper, at a terminal 38. Theproportioning resistor 36 includes a tap 39 for tapping voltage off ofthe resistor depending upon the percentage of copper in the particulartype of coil 11 under test, and connected to feed this voltage to adirect current buffer amplifier 41 having a gain of exactly one andpreferably of an emitter follower type requiring no adjustment. As willbe apparent to those skilled in the art, the proportioning resistor 36may be suitably calibrated in terms of the percentage of copper in thecoil 11 being tested, to facilitate the adjustment ofthe tap 39.

The voltage output of the buffer amplifier 41, which is equal to thevoltage at the tap 39 and of the same polarity, feeds by separate leadsto both a two stage direct current amplifier 42 and a junction terminal43 between two precision voltage dividcr resistors 44 and 46 of equalmagnitude, in the comparator circuit 22. The amplifier 42, which isadjustable (by circuitry not shown) so as to have a gain of exactly two,doubles the voltage without inversion and feeds it through the twoprecision voltage divider resistors 44 and 46, to ground. Thus, thevoltage appearing at the junction terminal 43 between the resistors 44and 46 always is equal to the voltage output of the buffer amplifier 41and the voltage at the voltage tap 39 on the linear proportioningresistor 36. In this connection, the buffer amplifier 41 isolates thetap 39 from the resistor 46 so that the resistor 46 has no effect on thevoltage appearing at'the tap. The buffer amplifier 41 also isolates thelinear proportioning resistor 36 from the amplifier 42 so thatadjustment of the tap 39 on the resistor, with the resultant changes inresistance between the tap and the terminals 37 and 38, will have noeffect on the gain of the amplifier and its gain always will beconstant.

The comparator circuit 22 now determines whether the resistance of thecoil 11 under test meets required tolerance limits. In this connection,an upper limit variable-voltage tap 47 on the resistor 44 and a lowerlimit variable-voltage tap 48 on the resistor 46 deliver to a pair ofcomparing amplifiers 49 and 51, voltages which are slightly higher andslightly lower than the optimum voltage at the junction terminal 43 andwhich represent upper and lower tolerance limits for the coil 11 undertest, respectively. The voltage developed across the coil 11 under testand appearing at the terminal 18 also feeds to the two comparingamplifiers 49 and 51. Accordingly, if the voltage developed across thecoil 11 is greater than the reference voltage delivered to the comparingamplifier 49 from the upper limit tap 47 on the resistor 44, thiscomparing amplifier will energize a suitable indicating device, such asa lamp 52, in a well-known manner. Similarly, if the voltage across thecoil 11 is less than the reference voltage delivered to the comparingamplifier 51 from the lower limit tap 48 on the resistor 46, thiscomparing amplifier will light an indicating lamp 53. lfneither of thelamps 52 and 53 is lighted, the resistance of the coil 11 being testedis within acceptable tolerance limits.

Preferably, the resistance values of the input resistors 26 and 33 andof the feedback resistors 28 and 32 in the temperature compensatingcircuit 21 are relatively high so as to limit the current flow in thiscircuit to several microamperes. This reduces the tendency for theresistivity of the temperature compensating circuit 21 to be affected byinternal heating because of excessive current flow therethrough, withthe resultant introduction of errors into the system. At the same time,of course, the resistance values should not be too great because if theimpedance of the system is too high there will be a tendency forelectrical noise to be introduced into it from adjacent electricalequipment. In this connection, favorable operating results have beenachieved utilizing resistors 26, 28 and 32 of 7.5 kilohms and a resistor33 of 10 kilohms, with the input resistor 33 and its associated inputresistor 34 adjusted to match the pure copper feedback resistor 32 at afixed reference temperature as indicated hereinabove. The linearproportioning resistor 36 and the voltage divider resistors 44 and 46are selected to present a light to normal current load for theamplifiers 27, 31 and 41 and in practice a resistance value of 10kilohms for each of these resistors has been found to be suitable.

OPERATION In preparing the apparatus for general use, the resistor 29 inthe first amplifying circuit 23 is adjusted so that the circuit has again of exactly minus one (unity); the input resistors 33 and 34 in theamplifying circuit 24 are adjusted so that their effective resistancematches the resistance of the pure copper feedback resistor 32 at asuitable reference temperature, such as 68 F; the amplifier 41, if anadjustable type is used, is adjusted so that it has a gain of exactlyone; and the amplifier 42 is adjusted so that it has a gain of exactlytwo. This circuitry, which normally needs no further adjustment, then issuitably enclosed so as to be inaccessible to the operatoi during testoperations.

In testing a particular type of the coils 11 which contain a certainpercentage of copper, the standard resistor 12 is adjusted to theoptimum resistance for the coils; the tap 39 is set on the linearproportioning resistor 36 in accordance with the percentage of copper inthe coils; and the taps 47 and 48 on the voltage divider resistors 44and 46 are set to represent upper and lower resistance tolerance limitsfor the coils, respectively. One of the coils 11 then is positioned inthe receptacle contacts 13 and 14, whereupon voltages are produced atthe terminals 17 and 18 which are of a magnitude proportional to theresistance of the standard resistor 12 and the resistance of the coil11, respectively.

The voltage at the terminal 17 drives both of the amplifying circuits 23and 24 of the temperature-compensating circuit 21, and since theamplifying circuit 23 has a constant gain of exactly minus one itsoutput voltage is exactly equal to the voltage developed across thestandard resistor 12, but of opposite polarity. However, since the gainof the second amplifying circuit 24 varies in response to change inambient tem perature (from the reference temperature of 68 F.) as adirect function of the temperature coefficient of resistance of copper,its output voltage is proportional to the voltage developed across thestandard resistor 12 as a direct function of the temperature coefficientof resistance of copper. Thus, the voltage appearing at the terminal 37is representative of the optimum resistance for the coil 11 if itcontained no copper; the voltage appearing at the terminal 38 isrepresentative of the optimum resistance for the coil 11 if it containedI percent copper; and the voltage at the tap 39 on the linearproportioning resistor 36 is representative of the optimum resistancefor the coil 11 in accordance with the percentage of copper which itactually contains.

The voltage at the tap 39 drives the buffer amplifier 41, which has again of exactly one, whereby its output voltage is equal to the voltageat the tap and of the same polarity. The output voltage of the bufferamplifier 41 then is delivered to both the junction terminal 43 betweenthe precision voltage divider resistors 44 and 46, and to the amplifier42, of the comparator circuit 22. The amplifier 42, which has a gain ofexactly two, doubles the voltage received from the buffer amplifier 41,without inversion, and feeds it through the voltage divider resistors 44and 46, to ground. Thus, the voltage appearing at the junction terminal43 between the resistors 44 and 46 is equal to the voltage output of thebuffer amplifier 41 and to the voltage appearing at the tap 39 on thelinear proportioning resistor 36.

The comparator circuit 22 then determines whether the coil 11 under testmeets the required tolerance limits. In this regard, the voltage taps 47and 48 on the voltage divider resistors 44 and 46 deliver to thecomparing amplifiers 49 and 51, voltages which are slightly higher andslightly lower, respectively, than the optimum voltage at the junctionterminal 43v If the voltage developed across the coil 11 and being fedto the comparing amplifiers 49 and 51 from the terminal 18 is greaterthan the voltage received by the amplifier 49 from the tap 47, theamplifier will light the indicating lamp 52. Similarly, if the voltagedeveloped across the coil 11 is less than the voltage received by theamplifier 51 from the tap 48, this amplifier will light the indicatinglamp 53. If neither of the lamps 52 and 53 is lighted, the resistance ofthe coil 11 is within the acceptable tolerance limits.

What is claimed is:

1. ln apparatus in which a voltage developed across an electricalcomponent and a voltage developed across a standard resistance are fedto a comparing means for determining whether the resistance of theelectrical component falls within desired limits, and in which thepercentage ofa specific electrically conducting material in theelectrical component may vary, the improvement which comprises:

temperature-compensating means for modifying the voltage developedacross the standard resistance in response to changes in temperature andas a direct function of the temperature coefficient of resistance of thespecific electrically conducting material, said temperature-compensatingmeans being adjustable over a preselected range in accordance with thepercentage of the specific electrically conducting material in theelectrical component and being connected between the standard resistanceand the company means so as to feed the modified voltage to thecomparing means for comparison with the voltage developed across theelectrical component.

2. In apparatus as recited in claim 1, the improvement which furthercomprises:

said temperature-compensating means being responsive to the voltagedeveloped across the standard resistance for developing voltages over arange from a voltage which is representative of an optimum resistancefor an electrical component containing zero percent of the specificelectrically conducting material to a voltage which is representative ofan optimum resistance for an electrical component containing percent ofthe specific electrically conducting material; and

means for feeding a voltage in accordance with the percentage of thespecific electrically conducting material in the electrical componentunder test, from said temperaturecompensating means to the comparingmeans.

3. ln apparatus in which a voltage developed across an electricalcomponent and a voltage developed across a standard resistance are fedto a comparing means for determining whether the resistance of theelectrical component falls within desired limits, and in which thepercentage of a specific electrically conducting material in theelectrical component may vary, the improvement which comprises:

temperature-compensating means for modifying the voltage developedacross the standard resistance in response to changes in temperature andas a direct function of the temperature coefficient of resistance of thespecific electrically conducting material, said temperature-compensatingmeans including first and second amplifying circuits connected inparallel;

means for feeding the voltage developed across the standard resistanceto both of said amplifying circuits;

said first amplifying circuit having a constant gain and having anoutput voltage representative of the voltage developed across thestandard resistance;

said first amplifying circuit having a gain which varies in response tochanges in temperature and as a direct function of the temperaturecoefficient of resistance of the specific electrically conductingmaterial; and having an output voltage which varies in proportion to thevoltage developed across the standard resistance as a direct function ofthe temperature coefficient of resistance of the specific electricallyconducting material;

a resistance connected across outputs of said amplifying circuits suchthat the voltages at successive points on said resistance vary as afunction of the temperature coefficient of resistance of the specificelectrically conducting material, from a voltage which is representativeof an optimum resistance for an electrical component containing apreselected lower percentage ofthe specific electrically conductingmaterial to a voltage which is representative of an optimum resistancefor an electrical component containing a preselected upper percentage ofthe specific electrically conducting material;

means for tapping voltage off of said resistance in accordance with thepercentage of the specific electrically conducting material in theelectrical component under test; and

means for feeding the tapped voltage to the comparing means.

4. In apparatus as recited in claim 3,

which further comprises:

said first amplifying circuit having a gain of unity and including aninput resistance and a feedback resistance of the same relatively highresistance value, said resistances each having a zero temperaturecoefficient of resistance; and

said second amplifying circuit including an input resistance and afeedback resistance of the same relatively high resistance value at afixed reference temperature, the input resistance having a zerotemperature coefficient of resistance and the feedback resistance beingmade of the specific electrically conducting material.

the improvement UNITED STATES PATENT OFFICE CERTIFICATE OF CORETION- A3, 33,098 Dated January L1,, 1972 R. Lo Westlund Patent No.

lnventor( s) It is certified rhat error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

r olmnn 5, line 13 "change" should read -changes. Column 5:

' should read comparing-m Column 6, line 37,

- line 75, "company "first" should read ""SBGODd-"'"o Signed and sealedthis 30th day of May 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK Commissioner of Patents EDWARD MJLETCHER, JR.Attesting Officer

1. In apparatus in which a voltage developed across an electricalcomponent and a voltage developed across a standard resistance are fedto a comparing means for determining whether the resistance of theelectrical component falls within desired limits, and in which thepercentage of a specific electrically conducting material in theelectrical component may vary, the improvement which comprises:temperature-compensating means for modifying the voltage developedacross the standard resistance in response to changes in temperature andas a direct function of the temperature coefficient of resistance of thespecific electrically conducting material, said temperature-compensatingmeans being adjustable over a preselected range in accordance with thepercentage of the specific electrically conducting material in theelectrical component and being connected between the standard resistanceand the comparing means so as to feed the modified voltage to thecomparing means for comparison with the voltage developed across theelectrical component.
 2. In apparatus as recited in claim 1, theimprovement which further comprises: said temperature-compensating meansbeing responsive to the voltage developed across the standard resistancefor developing voltages over a range from a voltage which isrepresentative of an optimum resistance for an electrical componentcontaining zero percent of the specific electrically conducting materialto a voltage which is representative of an optimum resistance for anelectrical component containing 100 percent of the specific electricallyconducting material; and means for feeding a voltage in accordance withthe percentage of the specific electrically conducting material in theelectrical component under test, from said temperature-compensatingmeans to the comparing means.
 3. In apparatus in which a voltagedeveloped across an electrical component and a voltage developed acrossa standard resistance are fed to a comparing means for determiningwhether the resistance of the electrical component falls within desiredlimits, and in which the percentage of a specific electricallyconducting material in the electrical component may vary, theimprovement which comprises: temperature-compensating means formodifying the voltage developed across the standard resistance inresponse to changes in temperature and as a direct function of thetemperature coefficient of resistance of the specific electricallyconducting material, said temperature-compensating means including firstand second amplifying circuits connected in parallel; meanS for feedingthe voltage developed across the standard resistance to both of saidamplifying circuits; said first amplifying circuit having a constantgain and having an output voltage representative of the voltagedeveloped across the standard resistance; said second amplifying circuithaving a gain which varies in response to changes in temperature and asa direct function of the temperature coefficient of resistance of thespecific electrically conducting material; and having an output voltagewhich varies in proportion to the voltage developed across the standardresistance as a direct function of the temperature coefficient ofresistance of the specific electrically conducting material; aresistance connected across outputs of said amplifying circuits suchthat the voltages at successive points on said resistance vary as afunction of the temperature coefficient of resistance of the specificelectrically conducting material, from a voltage which is representativeof an optimum resistance for an electrical component containing apreselected lower percentage of the specific electrically conductingmaterial to a voltage which is representative of an optimum resistancefor an electrical component containing a preselected upper percentage ofthe specific electrically conducting material; means for tapping voltageoff of said resistance in accordance with the percentage of the specificelectrically conducting material in the electrical component under test;and means for feeding the tapped voltage to the comparing means.
 4. Inapparatus as recited in claim 3, the improvement which furthercomprises: said first amplifying circuit having a gain of unity andincluding an input resistance and a feedback resistance of the samerelatively high resistance value, said resistances each having a zerotemperature coefficient of resistance; and said second amplifyingcircuit including an input resistance and a feedback resistance of thesame relatively high resistance value at a fixed reference temperature,the input resistance having a zero temperature coefficient of resistanceand the feedback resistance being made of the specific electricallyconducting material.